1. Field of the Invention
The present invention relates to a magnetic powder suitable for magnetic recording with high recording density and a magnetic recording medium containing the same for example a magnetic tape.
2. Description of the Related Art
Conventionally, a magnetic recording medium of coating type is made by coating a magnetic powder, in combination with a resin binder on a non-magnetic base material such as a polyethylene film. In recent years, as the need of high recording density of magnetic recording media increases, single phase magnetic particles of hexagonal ferrite in accordance with perpendicular magnetic recording system and magnetic recording medium using them have been developed. The magnetic recording media in accordance with the perpendicular magnetic recording system are mainly made by coating a hexagonal ferrite magnetic powder such as Ba ferrite dispersed in a resin binder on a base material. These powders are plate shaped and have an easily magnetizable axis perpendicular to the plate surface. Each particle of the plate shaped hexagonal ferrite of single phase has a crystal structure where two types of layer units referred to as spinel block (or S block) and R block are regularly layered.
A magnetic recording medium using a hexagonal ferrite magnetic powder in accordance with a perpendicular magnetic recording system can record data in higher density than a conventional magnetic recording medium using a needle shaped magnetic powder in accordance with longitudinal magnetic recording system.
This is because the hexagonal ferrite magnetic powder is made of very fine particles and the particles are so arranged in the magnetic layer, which is made by smoothly coating the particles on the base material with a high packing ratio, that magnetized direction of them are perpendicular to the medium surface and not magnetically repulsive one another. In addition, the magnetic recording media using the hexagonal ferrite powder can provide a higher reproduction output in a short wavelength range than the conventional medium in accordance with longitudinal magnetic recording system.
Recently, a magnetic particle where hexagonal ferrite and spinel structure ferrite are integrated has been proposed so as to enhance the saturation magnetization thereof, the saturation magnetization of the former being lower than that of the latter. This magnetic particle is referred to as a composite magnetic particle. In the crystal structure of the composite magnetic particle, S blocks are irregularly added to other S blocks of the hexagonal ferrite, where S blocks and R blocks are regularly layered. Thereby both S blocks and R blocks are irregularly layered in the crystal structure of this composite magnetic powder. Thus, this composite magnetic particle has been considered to possess both the crystal structure of the spinel ferrite and that of the hexagonal ferrite.
However, thus far it was very difficult to produce a magnetic recording medium which, by using the above mentioned magnetic particles in accordance with the perpendicular magnetic recording system, satisfies all electromagnetic characteristics such as reproducing output and signal-to-noise ratio (hereinafter, referred to as an S/N ratio).
For example, a magnetic powder of hexagonal ferrite is not easily dispersed to a resin binder in comparison with a needle shaped magnetic powder. Thus, so far the reproduction output of the magnetic recording medium produced by coating the magnetic powder of hexagonal ferrite is not higher than that expected. The magnetic particles of hexagonal ferrite tend to be stacked one another magnetically in the direction of their easily magnetizable axes which are vertical direction to the surface of plate shaped particles. Thus, the dispersion property of magnetic particles of hexagonal ferrite has been considered to be deteriorated by this magnetical stacking. As a result, it seems that the magnetic particles of hexagonal ferrite can not display their intrinsic ability. In addition, since the dispersion property is deteriorated, a smooth surface of magnetic layer cannot be easily formed by using hexagonal ferrite. Thus, the magnetic recording by using hexagonal ferrite involves relatively large noise. As described above, since the hexagonal ferrite can hardly be dispersed into the resin binder, the magnetic recording medium using the single phase magnetic particles thereof has not provided a large reproduction output and a high S/N ratio.